Effects of topography and soil properties on recharge at two sites in an agricultural field

Field experiments were conducted from 1992 to 1995 to estimate ground water recharge rates at two sites located within a 2.7-hectare agricultural fiel d. The field lies in a sand plain setting in central Minnesota and is cropp ed continuously in field corn. The sites are located at a topographically h igh (upland) site and a topographically low (lowland) site in an effort to quantify the effects of depression focusing of recharge. Three site-specifi c methods were used to estimate recharge rates: well hydrograph analysis, c hlorofluorocarbon age dating, and an unsaturated zone water balance. All th ree recharge methods indicated that recharge rates at the lowland site (ann ual average of all methods of 29 cm) exceeded those at the upland site (ann ual average of 18 cm). On an annual basis, estimates by the individual meth ods ranged from 12 to 44 percent of precipitation at the upland site and fr om 21 to 83 percent at the lowland site. The difference in recharge rates b etween the sites is primarily attributed to depression focusing of surface water runon at the lowland site. However, two other factors were also impor tant: the presence of thin lamellae at the upland site, and coarser texture d soils below a depth of 1.5 m at the lowland site.